Vehicle seat frame

ABSTRACT

Provided is a vehicle seat frame configured so that the weight of the frame can be reduced while the strength of a portion of the frame can be ensured. A vehicle seat frame F includes a frame member molded from a high tensile strength steel plate. A metal structure of a portion of the frame member is a structure formed according to one or more conditions upon a thermal treatment performed on the frame member, and the strength of the portion is different from those of other portions of the frame member. For example, the portion of the frame member includes a high strength portion with a higher strength than those of the other portions. Moreover, the portion of the frame member includes, for example, a low strength portion with a lower strength than those of the other portions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Entry application of PCTApplication No. PCT/JP2016/072216, filed Jul. 28, 2016, which claims thepriority benefit of Japanese Patent Application No. JP 2015-150056,filed Jul. 29, 2015, the contents being incorporated herein byreference.

BACKGROUND

The present disclosure relates to a vehicle seat frame, and particularlyrelates to a vehicle seat frame using a thin, lightweight material.

Weight reduction has been demanded for a vehicle seat frame (hereinafterreferred to as a “seat frame”) as a framework of a vehicle seat.However, even with weight reduction, the strength of the seat frameneeds to be maintained and improved.

Thus, in a technique described in Japanese Patent Publication JP 5657271B, thermal treatment by high-frequency quenching, liquid carburizing, orgas carburizing is performed for a frame member forming a seat framemolded using general steel or for the entirety of the seat frame, and inthis manner, the strength of the seat frame molded using the generalsteel is improved.

Moreover, in a technique described in Japanese Patent Publication JP5565785 B, thin steel has a reinforcement portion, and a high hardnessportion is provided at the thin steel by thermal treatment. Further, inthe technique described in Japanese Patent Publication JP 5565785 B, theshape of a force transmission path formed by the reinforcement portionand the high hardness portion is a substantially truss-like shape, andtherefore, the strength of a seat frame is improved.

For further weight reduction, a high tensile strength steel plate as athin lightweight material might be sometimes used for the frame memberforming the seat frame. However, molding of the high tensile strengthsteel plate is more difficult than that of the general steel. In thecase of using such a high tensile strength steel plate, when the thermaltreatment is performed for the frame member requiring strength or theentirety of the seat frame as in the technique described in JapanesePatent Publication JP 5657271 B, there is a probability that greatstrain is caused due to the thermal treatment, leading to lower productaccuracy. Moreover, in the case of providing the reinforcement portionat the thin steel as in the technique described in Japanese PatentPublication JP 5565785 B, a weight increases by the reinforcementportion, and therefore, there is a problem in weight reduction.

SUMMARY

The present disclosure has been made in view of the above-describedproblems, and various embodiments provide a vehicle seat frameconfigured so that the weight of the frame can be reduced while thestrength of the frame can be partially adjusted.

The above-described problems are solved by an embodiment of a vehicleseat frame according to the present disclosure. The vehicle seat frameincludes a frame member molded using a high tensile strength steelplate. A metal structure of a portion of the frame member is a structureformed according to one or more conditions upon a thermal treatmentperformed on the frame member, and the strength of the portion isdifferent from those of other portions of the frame member.

In the above-described vehicle seat frame, the metal structure of theportion of the frame member molded from the high tensile strength steelplate is the structure formed according to the conditions upon thethermal treatment, and the strength of the portion is different fromthose of the other portions. With this configuration, in theabove-described vehicle seat frame, the strength can be partiallychanged in the same frame member without changing a shape or a material.Thus, a thin lightweight material can be used for the frame member torealize weight reduction while the strength of the frame member can bepartially adjusted.

In the above-described vehicle seat frame, the portion may include ahigh strength portion with a higher strength than those of the otherportions. According to the above-described configuration, the strengthof a portion subjected to the thermal treatment can be, in the vehicleseat frame, higher than that of a portion not subjected to the thermaltreatment. That is, according to the above-described configuration,deformation of a portion of the frame member subjected to the thermaltreatment under predetermined conditions can be suppressed.

In the above-described vehicle seat frame, the high strength portion maybe a portion heated to a temperature equal to or higher than apredetermined temperature and then cooled at a cooling rate equal to orhigher than a predetermined cooling rate to form the metal structure.According to the above-described configuration, the strength of theportion of the frame member quenched after heating under thepredetermined conditions can be higher than those of the other portions.That is, according to the above-described configuration, the coolingrate for the portion of the frame member in the thermal treatment iscontrolled so that the portion having a high strength can be partiallyprovided at the frame member.

In the above-described vehicle seat frame, a metal structure of the highstrength portion may be martensite. According to the above-describedconfiguration, the portion whose metal structure is a high hardnessmartensite can be partially provided at the frame member. With thisconfiguration, the portion having a high strength can be partiallyprovided at the frame member.

In the above-described vehicle seat frame, the high strength portion maybe provided at at least one of i) the periphery of a welding portionbetween an upper frame and a side frame of a seat back frame, or ii) theperiphery of a welding portion between a lower frame and the side frameof the seat back frame. According to the above-described configuration,stiffness can be enhanced at the periphery of the welding portionbetween the upper frame and the side frame of the seat back frame or theperiphery of the welding portion between the lower frame and the sideframe of the seat back frame, stress tending to concentrate on theseperipheral portions upon vehicle collision. With this configuration,deformation of the vehicle seat frame can be suppressed.

In the above-described vehicle seat frame, the portion may include a lowstrength portion with a lower strength than those of the other portions.According to the above-described configuration, the portion having a lowstrength can be partially provided at the frame member without changingthe shape or material of the frame member forming the vehicle seatframe. With this configuration, deformation of the portion of the framemember subjected to the thermal treatment under the predeterminedconditions can be accelerated.

In the above-described vehicle seat frame, the low strength portion maybe a portion heated to a temperature equal to or higher than thepredetermined temperature and then cooled at a cooling rate lower thanthe predetermined cooling rate to form the metal structure. According tothe above-described configuration, the strength of the portion includedin the frame member forming the vehicle seat frame and slowly cooledafter heating under the predetermined conditions can be lower than thoseof the other portions. As described above, the cooling rate for theframe member in the thermal treatment is controlled so that the portionhaving a low strength can be partially provided at the frame member.

In the above-described vehicle seat frame, a metal structure of the lowstrength portion may include pearlite. Note that pearlite is a genericterm of a state in which ferrite and cementite coexist, and is acementite layer with a ferrite base, for example. According to theabove-described configuration, the portion whose metal structureincludes a low hardness pearlite is partially provided at the framemember so that the portion having a low strength can be partiallyprovided at the frame member.

In the above-described vehicle seat frame, the low strength portion maybe provided at at least one of i) a connection portion between the seatback frame and a seat cushion frame, ii) a lower portion of the sideframe forming the seat back frame, and iii) a back portion of a cushionside frame forming the seat cushion frame. Moreover, the above-describedconnection portion may be configured integrally with the cushion sideframe. According to the above-described configuration, the low strengthportion functions as a weak portion so that energy upon vehiclecollision can be efficiently absorbed.

In the above-described vehicle seat frame, the thermal treatment may beperformed with the frame member being assembled. According to theabove-described configuration, the magnitude of strain caused due to thethermal treatment can be suppressed as compared to the case ofperforming the thermal treatment before the frame member beingassembled.

In the above-described vehicle seat frame, the high strength portion maybe provided along a connection portion between a seat side surface and aseat back surface at the side frame of the seat back frame. According tothe above-described configuration, the strength of the seat back frameagainst force applied on a seat back side can be improved.

In the above-described vehicle seat frame, the side frame of the seatback frame may be provided with a plurality of holes for airbagattachment, and the high strength portion may be provided to extend in aseat front-to-back direction between the plurality of holes. Accordingto the above-described configuration, the strength of the side frame ofthe seat back frame against force applied in the seat front-to-backdirection can be improved. With this configuration, deformation of theside frame of the seat back frame can be suppressed.

In the above-described vehicle seat frame, the high strength portion mayhave a first region provided along the connection portion between theseat side surface and the seat back surface at the side frame of theseat back frame, and a plurality of second regions provided to extend inthe seat front-to-back direction between the plurality of holes.According to the above-described configuration, the strength can beimproved across a wide area of the seat side surface of the side frameof the seat back frame.

In the above-described vehicle seat frame, the high strength portion maybe provided along or provided overlapping with at least one of i) thewelding portion between the upper frame of the seat back frame and theside frame of the seat back frame, or ii) the welding portion betweenthe lower frame of the seat back frame and the side frame of the seatback frame. According to the above-described configuration, the strengthcan be improved across a wide area of the side frame of the seat backframe in a seat upper-to-lower direction.

In the above-described vehicle seat frame, the high strength portion maybe provided at a portion where the upper frame of the seat back frameand the side frame of the seat back frame overlap with each other.According to the above-described configuration, the strength of a jointportion between the upper frame and the side frame of the seat backframe can be improved.

According to the present disclosure, the weight of the vehicle seatframe can be reduced while the strength of a necessary portion of thevehicle seat frame can be ensured.

According to an embodiment of the present disclosure, deformation of theportion of the vehicle seat frame subjected to the thermal treatmentunder the predetermined conditions can be reduced.

According to an embodiment of the present disclosure, the cooling ratefor the portion of the vehicle seat frame in the thermal treatment iscontrolled so that the portion having a high strength can be partiallyprovided at the vehicle seat frame.

According to an embodiment of the present disclosure, the portion whosemetal structure is the high hardness martensite is partially provided atthe vehicle seat frame so that the portion having a high strength can bepartially provided at the vehicle seat frame.

According to an embodiment of the present disclosure, the stiffness canbe enhanced at the periphery of the welding portion between the upperframe and the side frame of the seat back frame or the periphery of thewelding portion between the lower frame and the side frame of the seatback frame, the stress tending to concentrate on these peripheralportions upon vehicle collision. Thus, deformation of the vehicle seatframe can be suppressed.

According to an embodiment of the present disclosure, the portion havinga low strength can be partially provided at the frame member withoutchanging the shape or material of the frame member forming the vehicleseat frame.

According to an embodiment of the present disclosure, the cooling ratefor the frame member in the thermal treatment is controlled so that theportion having a low strength can be partially provided at the framemember.

According to an embodiment of the present disclosure, the portion whosemetal structure includes the low hardness pearlite is partially providedat the frame member so that the portion having a low strength can bepartially provided at the frame member.

According to an embodiment of the present disclosure, the energy uponvehicle collision can be efficiently absorbed.

According to an embodiment of the present disclosure, the magnitude ofstrain caused due to the thermal treatment can be suppressed as comparedto the case of performing the thermal treatment before assembly.

According to an embodiment of the present disclosure, the strength ofthe seat back frame against the force applied on the seat back side canbe improved.

According to an embodiment of the present disclosure, the strength ofthe side frame of the seat back frame against the force applied in theseat front-to-back direction can be improved.

According to an embodiment of the present disclosure, the strength canbe improved across the wide area of the seat side surface of the sideframe of the seat back frame.

According to an embodiment of the present disclosure, the strength canbe improved across the wide area of the side frame of the seat backframe in the seat upper-to-lower direction.

According to an embodiment of the present disclosure, the strength ofthe joint portion between the upper frame and the side frame of the seatback frame can be improved.

BRIEF DESCRIPTION OF DRAWINGS

Various embodiments of the invention are illustrated in the drawings, inwhich:

FIG. 1 is a perspective view of a vehicle seat of the presentembodiment;

FIG. 2 is a perspective view of a seat frame as a framework of thevehicle seat;

FIG. 3 is a graph for describing a relationship among thermal treatmentconditions and a metal structure;

FIG. 4 is an enlarged view of an upper portion of FIG. 2, the view beinga perspective view of an example of a thermal treatment portion at ajoint portion between an upper frame and each side frame;

FIG. 5 is an enlarged view of the upper portion of FIG. 2, the viewbeing a perspective view of another example of the thermal treatmentportion at the joint portion between the upper frame and each sideframe;

FIG. 6 is a cross-sectional view along a VI-VI line of FIG. 5, the viewbeing a cross-sectional view of the thermal treatment portion at thejoint portion between the upper frame and the side frame;

FIG. 7 is an enlarged view of a joint portion between the side frame anda lower frame of FIG. 2, the view being a view of an example of athermal treatment portion at the joint portion;

FIG. 8 is a side view of an example of a thermal treatment portion at acoupling bracket;

FIG. 9 is an enlarged view of an example of thermal treatment portionsof a seat back frame and a cushion frame;

FIG. 10 is a perspective view of a seat back frame of a back seat;

FIG. 11 is an enlarged view of a portion XI of FIG. 9, the view being aperspective view of thermal treatment portions at a joint portion of aframe framework portion;

FIG. 12 is a perspective view of a vehicle seat frame of a variation ofthe present disclosure;

FIG. 13 is a side view of thermal treatment portions of the vehicle seatframe of the variation;

FIG. 14 is a perspective view from a view point XIV indicated by anarrow of FIG. 12; and

FIG. 15 is a perspective view from a view point XV indicated by an arrowof FIG. 12.

DETAILED DESCRIPTION

A vehicle seat and a seat frame as a framework of the vehicle seataccording to an embodiment of the present disclosure is described belowwith reference to FIGS. 1 to 15.

The present embodiment relates to an invention of a vehicle seat frameincluding a frame member molded from a high tensile strength steelplate, where a metal structure of a portion of the frame member has astructure according to conditions upon thermal treatment and that thestrength of the portion is different from those of other portions of thehigh tensile strength steel plate. Note that an occupant seating sidewith respect to a seat back of the vehicle seat is a seat front side,and an opposite side thereof is a seat back side.

As illustrated in FIGS. 1 and 2, a vehicle seat S of the presentembodiment mainly includes a seat body having a seat cushion 1 and aseat back 2, a reclining mechanism 20 rotatably coupling the seat back 2to the seat cushion 1, a height link mechanism 21 liftably coupling theseat body to a vehicle body floor, and a rail mechanism 40 attached tothe vehicle body floor and configured to support the seat body such thatthe seat body is movable back and forth.

The seat cushion 1 is a seating portion that upwardly supports a seatedoccupant. As illustrated in FIGS. 1 and 2, the seat cushion 1 isconfigured such that a cushion pad 1 a placed on a cushion frame 10 as aframework is covered with a skin material 1 b.

The seat back 2 is a backrest portion supporting the back of theoccupant from the back side. The seat back 2 is configured such that acushion pad 2 a placed on a seat back frame 30 as a framework is coveredwith a skin material 2 b.

A vehicle seat frame F is a framework of the vehicle seat S. Asillustrated in FIG. 2, the vehicle seat frame F includes the cushionframe 10, the seat back frame 30, and a coupling bracket 50 coupling thecushion frame 10 and the seat back frame 30 together.

As illustrated in FIG. 2, the cushion frame 10 includes a substantiallyrectangular frame body as the framework of the seat cushion 1. Thecushion frame 10 includes cushion side frames 11 arranged respectivelyon right and left sides, a plate-shaped pan frame 12 coupling frontportions of the cushion side frames 11 together, and a pipe-shaped backcoupling frame 13 coupling back portions of the cushion side frames 11together. Further, the cushion frame 10 includes three elastic springs14 hooked on the pan frame 12 and the back coupling frame 13 andextending in a serpentine-like shape, and a lower pipe (not shown)disposed below the pan frame 12 and coupling the cushion side frames 11together.

Each cushion side frame 11 is a sheet-metal member that extends in theseat front-to-back direction and has a substantially U-shapedlongitudinal cross-section. The reclining mechanism 20 is attached to aback portion of each cushion side frame 11, and the rail mechanism 40 isattached to a lower portion of the reclining mechanism 20 through theheight link mechanism 21.

The reclining mechanism 20 is switchable between a lock state that locksa rotation operation of the seat back 2 and an unlock state. The lockstate of the reclining mechanism 20 is changed to the unlock state by areclining operation lever that is operable from a state in which theseat back 2 is locked in a standing posture, and therefore, the standingposture of the seat back 2 can be adjusted.

The height link mechanism 21 is configured to adjust the height of theseat body. The height link mechanism 21 includes, between the cushionframe 10 and the rail mechanism 40, two links attached respectively tothe seat front side and the seat back side, and a height adjustmentmechanism is provided by operation of these links. Note that each of theright and left cushion side frames 11 includes two links.

The rail mechanism 40 mainly includes right and left lower rails 41fixed to the vehicle body floor and extending in the seat front-to-backdirection, right and left upper rails 42 slidably supported along thelower rails 41, a lock member configured to lock the upper rails 42 suchthat the upper rails 42 are not slidable, and a rail operation leverconfigured to unlock a lock state of the lock member.

The lower rails 41 include elongated hollow bodies, and are arrangedrespectively on the right and left sides with a spacing in a seat widthdirection. Each lower rail 41 forms, along the seat front-to-backdirection, a housing space having a substantially raised section.

Each upper rail 42 includes an elongated body slidably movable along acorresponding one of the lower rails 41 with the upper rail 42 beinginserted into the housing space of the lower rail 41. As illustrated inFIG. 2, a link connected to the cushion frame 10 through a rail couplingbracket 45 is coupled to an upper portion of each upper rail 42.

Next, a configuration of the seat back frame 30 is described in detail.As illustrated in FIG. 2, the seat back frame 30 includes an upper frame31 as a hollow cylindrical body provided on an upper side and formed ina substantially inverted U-shape, right and left side frames 32 coupledrespectively to right and left end portions of the upper frame 31, and alower frame 33 bridged between lower end portions of the right and leftside frames 32. Further, the seat back frame 30 includes an elasticspring 34 bridged between inner surfaces of the right and left sideframes 32, and an upper cross frame 36 bridged between inner surfaces ofcurved portions of the upper frame 31.

The right and left side frames 32 are each formed in a substantiallyU-like shape such that a sheet-metal member is bent inward in theright-to-left direction. The right and left side frames 32 are separatedfrom each other in the right-to-left direction with the side frames 32extending in the upper-to-lower direction and being substantiallyparallel to each other. Each side frame 32 has an airbag attachment hole82 a, an airbag attachment hole 82 b, and a through-hole 82 c as holesfor attachment of an airbag unit (not shown). The airbag unit includes,for example, an airbag body, a webbing configured to guide an expansiondirection of an airbag, and a plate for attachment of the airbag. Forexample, the airbag attachment hole 82 a and the airbag attachment hole82 b are portions to which the webbing of the airbag is attached, andthe through-hole 82 c is a portion into which a bolt extending from amodule of the airbag is inserted. Moreover, a bead portion is formed atthe periphery of the through-hole 82 c. In particular, a substantiallyU-shaped bead portion 83 is formed on the seat front side of thethrough-hole 82 c between the airbag attachment hole 82 a and the airbagattachment hole 82 b.

An inner surface of an upper end portion of the right side frame 32 anda right end portion of the upper frame 31 are arranged in contact witheach other. Moreover, the upper end portion of the right side frame 32and the right end portion of the upper frame 31 are fixed by welding.Similarly, an inner surface of an upper end portion of the left sideframe 32 and a left end portion of the upper frame 31 are arranged incontact with each other. Moreover, the upper end portion of the leftside frame 32 and the left end portion of the upper frame 31 are fixedby welding. Note that various types of welding such as laser welding andarc welding may be used as welding.

As illustrated in FIG. 2, the lower frame 33 includes a frame centerportion extending in the right-to-left direction, and end portionsformed in such a manner that portions extending from the frame centerportion toward the right and left sides are bent to face each other.

An inner surface of the right end portion of the lower frame 33 isdisposed in contact with the right side frame 32. Moreover, the lowerend portion of the right side frame 32 and the right end portion of thelower frame 33 are fixed by welding. Similarly, an inner surface of theleft end portion of the lower frame 33 is disposed in contact with theleft side frame 32. Moreover, the lower end portion of the left sideframe 32 and the left end portion of the lower frame 33 are fixed bywelding.

An end portion of each side frame 32 has a shaft through-holepenetrating in the right-to-left direction. A coupling shaft 35penetrates the reclining mechanism 20, the coupling bracket 50 couplingthe reclining mechanism 20 and the cushion frame 10 together, and thelower end portions of the right and left side frames 32, thereby fixingeach portion. Moreover, the coupling shaft 35 serves as a rotary shaftwhen the seat back frame 30 is rotated by the reclining mechanism 20.

In the seat frame of the present embodiment, the thermal treatment of astrength adjustment portion of the seat frame for performing a partialhardness adjustment is performed in a state in which a frame membermolded mainly using a high tensile strength steel plate (e.g., a 440megapascals (MPa) material) is assembled by welding etc. In the presentembodiment, the above-described thermal treatment is performed byheating treatment using a laser and cooling treatment for a heatedportion, but the present disclosure is not limited to such treatment.Moreover, the above-described frame member is an element forming theseat frame. For example, each member forming the seat back frame 30,each member forming the cushion frame 10, and the bracket coupling theseat back frame 30 and the cushion frame 10 together are examples of theabove-described frame member.

Specifically, the thermal treatment (the thermal treatment for strengthimprovement) is performed such that strength is increased at theperiphery of a welding portion between the upper frame 31 and each sideframe 32 and the periphery of a welding portion between each side frame32 and the lower frame 33. The above-described thermal treatment forstrength improvement is performed in such a manner that the heatingtreatment and a cooling rate in the subsequent cooling treatment arecontrolled. Note that a portion whose metal structure (e.g.,microstructure) has been changed by the thermal treatment for strengthimprovement is referred to as a “high strength portion.”

Moreover, in the seat frame of the present embodiment, the thermaltreatment (the thermal treatment for strength reduction) is performedsuch that the strength is decreased at a portion of the coupling bracket50 coupling the cushion frame 10 and the seat back frame 30 together,the periphery of each lower end portion of the right and left sideframes 32 forming the seat back frame 30, or the periphery of each backend portion of the cushion side frames 11 forming the cushion frame 10.The above-described thermal treatment for strength reduction isperformed in such a manner that the heating treatment and the coolingrate in the subsequent cooling treatment are controlled. Note that aportion whose metal structure has been changed by the thermal treatmentfor strength reduction is referred to as a “low strength portion.” Arelationship among thermal treatment conditions (e.g., cooling rates)and a steel structure change is described below.

FIG. 3 shows classification of metal (steel) microstructurestransitioning from an austenite structure (S2) to other, differentmicrostructures, due to cooling rate differences, after steel has beenheated to a temperature equal to or higher than an A1 transformationpoint (heated to 1000° C., in the example shown in FIG. 3), andaccordingly, has been transformed into the austenite structure.

As shown in FIG. 3, when the steel with the austenite structure at 1000°C. is cooled at such a cooling rate (a “critical” cooling rate) that thesteel reaches a temperature equal to or lower than 540° C. within twoseconds (e.g., a cooling curve C1), the austenite structure transitionsto a martensite structure (S1).

Moreover, as shown in FIG. 3, when the steel with the austenitestructure at 1000° C. is cooled at a cooling rate lower than the coolingrate at which the steel reaches 540° C. within two seconds (e.g., acooling curve C2 or C3), the austenite structure transitions to a metalstructure that includes a pearlite structure (S3, S4). In this case,when the cooling curve is C2, the martensite structure and the pearlitestructure are mixed together (S3). When the cooling rate is lower forcooling with the cooling curve C3, this results in the pearlitestructure (S4).

Note that the relative hardness of the metal structure satisfies thefollowing:martensite (S1)>martensite+pearlite (S3)>pearlite (S4).

For example, a method in which dry ice powder is sprayed onto a thermaltreatment portion, a method in which cooled air is sprayed onto thethermal treatment portion, and a method in which dry mist is sprayedonto the thermal treatment portion may be used as the method for coolingthe steel heated to change into the austenite structure. Among thesemethods, cooling using dry mist is preferable for satisfying a coolingrate equal to or higher than the “critical” cooling rate (cooling curveC1). For example, at a cooling step using dry mist, low-temperature drymist containing a slight amount of moisture in cooled air may beutilized. In this case, a mist particle size is preferably about 30 μm,for example. At a quenching step, the above-described low-temperaturedry mist is used so that the amount of moisture used for cooling can bereduced while efficient cooling can be performed. Thus, quenching can bealso performed for the periphery of a component suspected to be damageddue to water, such as an electronic device. Moreover, the amount ofmoisture used for cooling can be reduced, and a mostly waterproofmechanism is not necessarily employed. Thus, a cost of the quenchingstep can be reduced.

In the present embodiment, in the case of increasing the strength by thethermal treatment, the cooling rate is controlled such that the metalstructure of the thermal treatment portion is transformed intomartensite. On the other hand, in the case of decreasing the strength bythe thermal treatment, the cooling rate is controlled such that themetal structure of the thermal treatment portion is transformed toinclude pearlite.

Next, the specific thermal treatment portion of the vehicle seat frame Fof the present embodiment is described with reference to FIGS. 4 to 7.First, a specific example of a formation portion of the high strengthportion formed in such a manner that the thermal treatment forincreasing the strength is performed for a portion of the vehicle seatframe F is described.

FIG. 4 is an enlarged view of a main portion of FIG. 2, the view being aperspective view of an example of a high strength portion formed at ajoint portion between the upper frame 31 and each side frame 32. In FIG.4, the upper frame 31 and the upper cross frame 36 are welded(arc-welded) at a welding portion 60 a. Moreover, the upper frame 31 andeach side frame 32 are welded (laser-welded) at welding portions 60 b,60 c. Note that the form of welding has been set forth as an example,and is not limited to the form of welding of the present embodiment asillustrated in the figure.

Each high strength portion 61 illustrated in FIG. 4 is provided in aregion including at least a portion of the welding portions 60 a, 60 b,60 c formed between the upper frame 31 and the side frame 32. The highstrength portion 61 described herein may be provided across the entirecircumferences of sections of the upper frame 31 and the side frame 32,or may be provided partially at each section.

Note that the following thermal treatment for strength improvement isperformed for each high strength portion 61. That is, in the thermaltreatment for strength improvement, the steel transformed into theaustenite structure by heating to 1000° C. is changed into themartensite structure by quenching to a temperature equal to or lowerthan 540° C. within two seconds. Thus, the hardness of the high strengthportion 61 subjected to the above-described thermal treatment forstrength improvement becomes higher than that of a portion not subjectedto the above-described thermal treatment for strength improvement. Thus,the strength of the joint portion between the upper frame 31 and theside frame 32 can be locally improved.

Next, another example of the high strength portion formed at the jointportion between the upper frame 31 and each side frame 32 is describedwith reference to FIGS. 5 and 6. FIG. 5 is an enlarged view of the mainportion of FIG. 2, the view being a perspective view of another exampleof the high strength portion formed at the joint portion between theupper frame 31 and each side frame 32. Moreover, FIG. 6 is across-sectional view along a VI-VI line of FIG. 5, the view being across-sectional view of the high strength portion formed at the jointportion between the upper frame 31 and the side frame 32.

As illustrated in FIGS. 5 and 6, each high strength portion 61 a has anarrower area than that of the high strength portion 61 illustrated inFIG. 4. That is, the high strength portion 61 a is provided in a regionincluding the welding portion between the upper frame 31 and the sideframe 32, but not including the welding portion between the upper crossframe 36 and the upper frame 31. Moreover, as illustrated in FIG. 6, thehigh strength portion 61 a is provided at seat-front-side portions ofthe sections of the upper frame 31 and the side frame 32, and is notnecessarily provided at seat-back-side portions.

Note that the thermal treatment for strength improvement is alsoperformed for the high strength portion 61 a such that a metal structureof the high strength portion 61 a is transformed into martensite. Thus,in the example illustrated in FIGS. 5 and 6, the strength of theperiphery of the joint portion between the upper frame 31 and the sideframe 32 on the seat front side on which stress concentrates uponvehicle collision can be locally improved.

Next, as illustrated in FIG. 7, a high strength portion may be providedat a joint portion between the side frame 32 and the lower frame 33, forexample. FIG. 7 is an enlarged view of a joint portion between the sideframe 32 and the lower frame 33 of FIG. 2, the view being a perspectiveview of an example of the high strength portion formed at the jointportion. In FIG. 7, the side frame 32 and the lower frame 33 are welded(laser-welded) at a welding portion 70.

The high strength portion 71 illustrated in FIG. 7 is provided in aregion including the welding portion 70 between the side frame 32 andthe lower frame 33. In this example, for the high strength portion 71,the thermal treatment for strength improvement is performed, in whichthe steel transformed into the austenite structure by heating to 1000°C. is changed into the martensite structure by quenching to atemperature equal to or lower than 540° C. within two seconds. Thus, thehardness of the high strength portion 71 becomes higher than that of aportion not subjected to the above-described thermal treatment forstrength improvement, and therefore, the strength of the joint portionbetween the side frame 32 and the lower frame 33 can be improved.

Next, a specific example of a formation portion of the low strengthportion formed at the frame member forming the vehicle seat frame F bythe thermal treatment for decreasing the strength is described withreference to FIGS. 8 and 9.

As illustrated in FIG. 8, a low strength portion 54 is provided at aportion of the coupling bracket 50 coupling the cushion frame 10 and theseat back frame 30 together. The coupling bracket 50 has bolt fasteningholes 52, 53 into each of which a bolt for fastening the cushion sideframe 11 and the coupling bracket 50 together is inserted and with ashaft insertion hole 51 for the coupling shaft 35. The recliningmechanism 20 is attached to the coupling shaft 35, and the seat backframe 30 is coupled to the coupling bracket 50 through the recliningmechanism 20.

In the coupling bracket 50 illustrated in FIG. 8, the low strengthportion having a lower strength than that of the periphery of the lowstrength portion is provided between a portion coupled to the cushionframe 10 and a portion coupled to the seat back frame 30. That is, inthe coupling bracket 50 illustrated in FIG. 8, the low strength portion54 whose metal structure has been changed, by thermal treatment, into,e.g., the pearlite structure with a lower hardness than the periphery ofthe low strength portion 54 is formed between the shaft insertion hole51 and the bolt fastening hole 53.

The thermal treatment for strength reduction as described below isperformed for the low strength portion 54. That is, in the thermaltreatment for strength reduction, the steel transformed into theaustenite structure by heating to 1000° C. with the laser is slowlycooled at a cooling rate lower than the critical cooling rate by, e.g.,natural cooling (self-cooling), thereby changing into the pearlitestructure. Note that in the case of using a high tensile strength steelplate with a strength equal to or higher than that of a 780 material (atensile strength of 780 MPa), the hardness of the low strength portion54 is, by the above-described thermal treatment for strength reduction,decreased as compared to those of metal structures of a portionsubjected to the thermal treatment for strength improvement and otherportions not subjected to the thermal treatment, and therefore, thestrength of the coupling bracket 50 can be partially decreased. Uponvehicle collision, the low strength portion 54 as a weak portion deformsso that stress concentration on the seat back frame 30 can be reduced.That is, the low strength portion 54 functions as the weak portion sothat impact upon vehicle collision can be efficiently absorbed.

Moreover, as illustrated in FIG. 9, a low strength portion 72 may beprovided at the periphery of the lower end portion of the side frame 32forming the seat back frame 30. The lower end portion of the side frame32 as described herein indicates the end portion of the side frame 32closer to the joint portion between the side frame 32 and the lowerframe 33. Note that the low strength portion 72 is also formed by thethermal treatment for strength reduction as described above. Asdescribed above, the low strength portion 72 is provided at theperiphery of the lower end portion of the side frame 32, and the lowstrength portion 72 as the weak portion deforms upon vehicle collision.Thus, impact upon vehicle collision can be efficiently absorbed.

Further, as illustrated in FIG. 9, a low strength portion 73 is providedat the periphery of the back end portion of the cushion side frame 11forming the cushion frame 10. The back end portion of the cushion sideframe 11 as described herein indicates the seat-back-side end portion ofthe cushion side frame 11. Note that the low strength portion 73 is alsoformed by the thermal treatment for strength reduction as describedabove. As described above, the low strength portion 73 is provided atthe periphery of the back end portion of the cushion side frame 11, andthe low strength portion 73 as a weak portion deforms upon vehiclecollision. Thus, impact upon vehicle collision can be efficientlyabsorbed. Note that in the case of forming any one of theabove-described low strength portions 54, 72, 73, the other low strengthportions are not necessarily formed.

In the vehicle seat frame of the above-described embodiment, the thermaltreatment portion is transformed into the austenite structure by heatingto a temperature equal to or higher than a predetermined temperature (atemperature equal to or higher than the A1 transformation point, such as1000° C.) with the laser, and subsequently, the cooling rate for such aportion is controlled such that the thermal treatment portiontransitions to the metal structure with the different strength. Thus,the strength of the thermal treatment portion of the vehicle seat framecan be differentiated from those of other portions without changing theshape, material, etc. of the vehicle seat frame. Moreover, in thevehicle seat frame of the above-described embodiment, control of thecooling rate is, after the heating treatment, is performed for a targetportion for strength adjustment (i.e., a strength adjustment targetportion of a frame comp product) in a state in which the frame membersforming the vehicle seat frame are assembled by, e.g., welding. In thismanner, the strength of the target portion can be adjusted.

Further, in the vehicle seat frame of the above-described embodiment,the target portion of the vehicle seat frame is heated to a temperatureequal to or higher than 1000° C. with the laser, and then, is cooled ata cooling rate equal to or higher than a predetermined cooling rate(e.g., equal to or higher than 230° C. per second). In this manner, thethermal treatment portion transitions, for example, from the austenitestructure to the martensite structure. Thus, the strength of the targetportion can be increased without a shape change, a thickness change,reinforcement member addition, a material change, etc. With thisconfiguration, in the seat frame molded from the high tensile strengthsteel plate, a strength (equal to or higher than 980 MPa) equivalent tothat of an extra-high tensile strength steel plate can be partiallyobtained.

In addition, in the vehicle seat frame of the above-describedembodiment, cooling is performed at a cooling rate lower than thepredetermined cooling rate (e.g., natural cooling) after heating withthe laser. The thermal treatment portion transitions, for example, fromthe austenite structure to the pearlite structure so that the strengthof a necessary portion can be decreased without a shape change, athickness change, processing of a raised-recessed portion, a cutout,etc., a material change, etc.

Moreover, the step of forming the high strength portions and the lowstrength portions at the seat frame by the thermal treatment can also beperformed after assembly of the frame members, leading to a high degreeof freedom in a manufacturing step. In the case of using the laser toweld the frame members forming the seat frame, the thermal treatmentportion can be heated using a facility, a jig, or a manufacturing linefor laser welding. Thus, a manufacturing cost can be reduced, leading toeasy mass production.

Further, in the vehicle seat frame of the above-described embodiment, aplanar portion for, e.g., component attachment is easily ensured, andthere are no surface protrusions and cutouts. This leads to a compactshape.

In addition, there are no specific limitations on a component targetedfor strength adjustment by the thermal treatment according to thepresent disclosure, and such a component may be any one of a pipe and apress product.

Moreover, in the vehicle seat frame of the above-described embodiment,an oxide layer remains as a manufacturing mark after laser heating. Evenif the oxide layer is coated for erasing the mark, checking by means ofan electronic microscope or handy energization can be performed.

Other Embodiments

The vehicle seat frame of the present disclosure is similarly applicablenot only to a front seat frame but also to a back seat frame.

FIG. 10 is a perspective view of a rear back frame 100 of a back seat.FIG. 11 illustrates an enlarged view of a main portion of FIG. 10, theview being a perspective view of a thermal treatment portion at a jointportion of a frame framework portion.

As illustrated in FIG. 10, the rear back frame 100 includes rectangularrod-shaped framework frames (e.g., framework frames 101, 102, 103)forming a framework of the rear back frame 100, and a plate-shaped panframe (e.g., a rear pan frame 105) forming a surface of the rear backframe 100. The framework frame (e.g., the framework frame 101) servingas a framework in a transverse direction of a seat back frame and theframework frames (e.g., the framework frames 102, 103) serving asframeworks in a longitudinal direction of the seat back frame are weldedtogether. In an example illustrated in FIG. 10, the frames arelaser-welded together, but may be joined together by other types ofwelding such as arc welding.

FIG. 11 illustrates enlarging a joint portion XI between the frameworkframe 101 and the framework frame 103 in the rear back frame 100illustrated in FIG. 10. As illustrated in FIG. 11, the framework frame101 and a flange portion 103 a of the framework frame 103 are weldedtogether at a welding portion 110 a. The framework frame 101 and aflange portion 103 b of the framework frame 103 are welded together at awelding portion 110 b. The framework frame 101 and a recessed portion103 c of the framework frame 103 are welded at a welding portion 110 c.

As illustrated in FIG. 11, in the rear back frame 100 according to oneembodiment of the present disclosure, high strength portions 120 ofridge portions of the framework frame 103 forming the rear back frame100 are subjected to thermal treatment for strength improvement. Forexample, in the thermal treatment for strength improvement, steeltransformed into an austenite structure by heating to 1000° C. ischanged into a martensite structure by quenching to a temperature equalto or lower than 540° C. within two seconds. In this manner, thehardness of each high strength portion 120 becomes higher than those ofmetal structures of a portion subjected to thermal treatment forstrength reduction and other portions not subjected to the thermaltreatment. Thus, the strength of the framework frame 103 can beimproved. Moreover, the strengths of the framework frames and the panframe other than the framework frame 103 may be partially adjusted bythe thermal treatment for strength improvement or the thermal treatmentfor strength reduction.

In the above-described embodiment, the vehicle seat S is configured toinclude the height link mechanism 21 and the rail mechanism 40 asillustrated in FIG. 2, but does not necessarily have this configuration.For example, a base member fixed to the vehicle body floor and thecushion frame 10 (the cushion side frames 11) may be directly coupledtogether. Moreover, the coupling bracket 50 may be formed integrallywith the cushion side frames 11, and in this manner, the number ofcomponents may be reduced.

Variation of Seat Back Frame

Next, a vehicle seat frame F1 of a variation of the present disclosureis described with reference to FIGS. 12 to 15. The vehicle seat frame F1is mainly different from the vehicle seat frame F in the configurationof the side frame 32 and the thermal treatment portion. Regarding thevehicle seat frame F1, differences from the vehicle seat frame F aremainly described below.

As illustrated in FIGS. 12 and 13, the side frames 32 of the vehicleseat frame F1 are joined to the upper frame 31 at welding portions 80and welding portions 81. The welding portions 80 are substantiallyU-shaped welding portions provided at two portions of the upper endportions of the side frames 32. The welding portions 80 join the upperend portions of the side frames 32 and the upper frame 31 together.Moreover, each welding portion 81 is a linear welding portion providedbelow a corresponding one of the welding portions 80. Each weldingportion 81 joins a lower end portion of the upper frame 31 and acorresponding one of the side frames 32 together.

Further, as illustrated in FIG. 15, each side frame 32 of the vehicleseat frame F1 is joined to the lower frame 33 at a welding portion 85 a,a welding portion 85 b, a welding portion 85 c, and a welding portion 85d. Note that an example where laser welding is used as welding of theupper frame 31, the side frames 32, and the lower frame 33 in thevehicle seat frame F1 is illustrated, but other welding methods such asarc welding may be used.

In addition, each side frame 32 of the vehicle seat frame F1 is, as inthe vehicle seat frame F, provided with the airbag attachment hole 82 a,the airbag attachment hole 82 b, and the through-hole 82 c as the holesfor attachment of the airbag unit (not shown). The airbag unit includes,for example, the airbag body, the webbing configured to guide theexpansion direction of the airbag, and the plate for attachment of theairbag. For example, the airbag attachment hole 82 a and the airbagattachment hole 82 b are the portions to which the webbing of the airbagis attached, and the through-hole 82 c is the portion into which thebolt extending from the module of the airbag is inserted.

Next, the thermal treatment portion of the vehicle seat frame F1 isdescribed with reference to FIGS. 13 to 15.

As illustrated in FIGS. 13 to 15, quenching for strength improvement is,for the vehicle seat frame F1, performed along a connection portion 32 cas a ridge portion connecting a seat side surface 32 a and a seat backsurface 32 b of the side frame 32, and in this manner, a high strengthportion 84 d and a high strength portion 84 e (equivalent to a firstregion) are formed. The high strength portion 84 d described herein isformed at a position facing the welding portion 81 at an upper portionof the connection portion 32 c. For example, the high strength portion84 d is formed above the airbag attachment hole 82 a. Moreover, the highstrength portion 84 e is, at a lower portion of the connection portion32 c, formed at a position at least partially overlapping with thewelding portion 85 a. For example, the high strength portion 84 e isformed below the airbag attachment hole 82 b. As described above, theside frame 32 has the high strength portion 84 d and the high strengthportion 84 e, and therefore, the strength of the side frame 32 againstforce applied to the seat back side can be improved. Note that in thevehicle seat frame F1, the high strength portion 84 d and the highstrength portion 84 e are formed separately from each other, but may beconnected as one.

Further, in the vehicle seat frame F1, quenching for strengthimprovement is performed for a region extending from the vicinity of ahigh strength portion 84 c toward the seat front side between the airbagattachment hole 82 b and the through-hole 82 c, and in this manner, ahigh strength portion 84 a (equivalent to a second region) is formed. Inaddition, in the vehicle seat frame F1, quenching for strengthimprovement is performed for a region extending from the vicinity of thehigh strength portion 84 c toward the seat front side between the airbagattachment hole 82 a and the through-hole 82 c, and in this manner, thehigh strength portion 84 b (equivalent to a second region) is formed. Asdescribed above, the high strength portion 84 a and the high strengthportion 84 b are formed at the side frame 32, and therefore, thestrength of the side frame 32 against force applied in the seatfront-to-back direction can be improved. This can reduce deformation ofthe side frame 32.

Moreover, quenching for strength improvement is performed for a region(a third region) connecting seat-front-side end portions of the highstrength portion 84 a and the high strength portion 84 b together, andin this manner, the high strength portion 84 c is formed. Quenching isperformed in different directions as described above, and therefore, thestrength can be improved with a proper balance across a wide area of theseat side surface 32 a of the side frame 32. Note that in the vehicleseat frame F, the bead portion 83 is provided on the front side of thethrough-hole 82 c, and therefore, stiffness is enhanced at the peripheryof the through-hole 82 c. On the other hand, in the vehicle seat frameF1, the substantially U-shaped quenched portion including the highstrength portion 84 a, the high strength portion 84 b, and the highstrength portion 84 c is provided on the front side of the through-hole82 c, and therefore, the stiffness can be enhanced at the periphery ofthe through-hole 82 c as in the bead portion 83.

In the vehicle seat frame F1, the seat side surface 32 a of each sideframe 32 is reinforced by the high strength portion 84 a, the highstrength portion 84 b, the high strength portion 84 c, the high strengthportion 84 d, and the high strength portion 84 e, and therefore, thestrength can be improved across a wide area of the seat side surface 32a.

Moreover, in an example illustrated in FIGS. 13 to 15, the high strengthportion 84 a and the high strength portion 84 b are formed at positionsseparated from the high strength portion 84 e and the high strengthportion 84 d. Note that the high strength portion 84 a and the highstrength portion 84 e may be coupled together, and the high strengthportion 84 b and the high strength portion 84 d may be coupled together.

Further, in the vehicle seat frame F1, the high strength portion 84 d isformed along (e.g., substantially in parallel to) the welding portion81, and the high strength portion 84 e is formed overlapping with thewelding portion 85 a extending in the seat upper-to-lower direction. Thehigh strength portion 84 d and the high strength portion 84 e along theconnection portion 32 c are formed as described above, and therefore,the strength can be improved across a wide area from an upper portion toa lower portion of each side frame 32.

In addition, in the vehicle seat frame F1, the high strength portion 84d is provided at a portion where the side frame 32 and the upper frame31 overlap with each other. Thus, the strength of the joint portionbetween the upper frame 31 and the side frame 32 can be improved.

In the above-described embodiments, the vehicle seat used for anautomobile has been described as a specific example, but the presentdisclosure is not limited to such a vehicle seat. The present disclosureis also applicable not only to vehicle seats for trains, buses, etc.,but also to conveyance seats for airplanes, ships, etc.

Moreover, in the above-described embodiments, e.g., a steel plate with atensile strength of equal to or higher than 340 MPa may be used as thehigh tensile strength steel plate, but a specification of the hightensile strength steel plate is not limited to the above examples.

In the present embodiments, the vehicle seat frame of the presentdisclosure has been mainly described. Note that the above-describedembodiments have been set forth as examples for the sake of easyunderstanding of the present disclosure, and are not intended to limitthe present disclosure. Changes and modifications can be made to thepresent disclosure without departing from the gist of the presentdisclosure, and needless to say, the present disclosure includesequivalents thereof.

REFERENCE SIGNS LIST

-   1: seat cushion    -   1 a, 2 a: cushion pad    -   1 b, 2 b: skin material-   2: seat back-   10: cushion frame-   11: cushion side frame-   12: pan frame-   13: back coupling frame-   14, 34: elastic spring-   20: reclining mechanism-   21: height link mechanism-   30: seat back frame-   31: upper frame-   32: side frame    -   32 a: seat side surface    -   32 b: seat back surface    -   32 c: connection portion-   33: lower frame-   35: coupling shaft-   36: upper cross frame-   40: rail mechanism-   41: lower rail-   42: upper rail-   45: rail coupling bracket-   50: coupling bracket-   51: shaft insertion hole-   52, 53: bolt fastening hole-   61, 61 a, 71, 120: high strength portion-   54, 72, 73: low strength portion-   60 a, 60 b, 60 c, 70, 110 a, 110 b, 110 c: welding portion (welding    mark)-   65 a, 65 b, 75 a: welding portion-   80: welding portion (welding mark)-   81: welding portion (welding mark)-   82 a: airbag attachment hole-   82 b: airbag attachment hole-   82 c: through-hole-   83: bead portion-   84 a: high strength portion (second region)-   84 b: high strength portion (second region)-   84 c: high strength portion (third region)-   84 d: high strength portion (first region)-   84 e: high strength portion (first region)-   85 a: welding portion (welding mark)-   85 b: welding portion (welding mark)-   85 c: welding portion (welding mark)-   85 d: welding portion (welding mark)-   100: rear back frame-   101, 102, 103: framework frame    -   103 a, 103 b: flange portion    -   103 c: recessed portion-   105: rear pan frame-   C1, C2, C3: cooling curve-   F: vehicle seat frame-   F1: vehicle seat frame-   S: vehicle seat

The invention claimed is:
 1. A vehicle seat frame comprising: a firstframe member of the vehicle seat frame and a second frame member of thevehicle seat frame, each of the first frame member and the second framemember being molded from a high tensile strength steel plate, whereineach of the first frame member and the second frame member includes arespective high strength portion having a higher strength than those ofother portions of the corresponding member, the high strength portionshaving a metal structure that has been changed by a thermal treatmentfor strength improvement, the first frame member and the second framemember are welded together at a welding portion to be joined together,and the high strength portions are formed at a joint portion at whichthe first frame member and the second frame member are joined together,and overlap with at least a portion of a welding mark of the weldingportion.
 2. The vehicle seat frame of claim 1, wherein the high strengthportion is a portion heated to a temperature equal to or higher than apredetermined temperature and then cooled at a cooling rate equal to orhigher than a predetermined cooling rate to form the metal structure. 3.The vehicle seat frame of claim 1, wherein the metal structure of thehigh strength portion includes martensite.
 4. The vehicle seat frame ofclaim 1, the vehicle seat frame comprising: a seat back frame thatincludes an upper frame, a lower frame, and a side frame; wherein thefirst frame member includes at least one of the upper frame or the lowerframe, the second frame member is the side frame, and the high strengthportion is provided at least one of i) a periphery of a first weldingportion between the upper frame and the side frame of the seat backframe, or ii) a periphery of a second welding portion between the lowerframe and the side frame of the seat back frame.
 5. The vehicle seatframe of claim 1, further comprising: a low strength portion having alower strength than those of other portions in the vehicle seat frame.6. The vehicle seat frame of claim 5, wherein the low strength portionis a portion heated to a temperature equal to or higher than apredetermined temperature and then cooled at a cooling rate lower than apredetermined cooling rate to change a metal structure of the lowstrength portion.
 7. The vehicle seat frame of claim 5, wherein themetal structure of the low strength portion includes pearlite.
 8. Thevehicle seat frame of claim 5, wherein the low strength portion isprovided at least one of i) a connection portion between a seat backframe of the vehicle seat frame and a seat cushion frame of the vehicleseat frame, ii) a lower portion of a side frame forming the seat backframe, and iii) a back portion of a cushion side frame forming the seatcushion frame.
 9. The vehicle seat frame of claim 1, wherein the thermaltreatment is performed with the first frame member and the second framemember being assembled.
 10. The vehicle seat frame of claim 4, whereinthe high strength portion is provided along a connection portion betweena seat side surface and a seat back surface at the side frame of theseat back frame.
 11. The vehicle seat frame of claim 4, wherein the sideframe of the seat back frame is provided with a plurality of holes forairbag attachment, and the high strength portion is provided to extendin a seat front-to-back direction between the plurality of holes. 12.The vehicle seat frame of claim 11, wherein the high strength portionhas a first region provided along a connection portion between a seatside surface and a seat back surface at the side frame of the seat backframe, and a plurality of second regions provided to extend in the seatfront-to-back direction between the plurality of holes.
 13. The vehicleseat frame of claim 4, wherein the welding mark is at least one of i) afirst welding mark that is between the upper frame of the seat backframe and the side frame of the seat back frame, or ii) a second weldingmark that is between the lower frame of the seat back frame and the sideframe of the seat back frame.
 14. The vehicle seat frame of claim 4,wherein the high strength portion is provided at a portion where theupper frame of the seat back frame and the side frame of the seat backframe overlap with each other.